This invention relates to a self-luminescent light source which provides a night display of a liquid crystal display watch.
A field effect type liquid crystal display element displays a character pattern by reflecting light from the outside with a reflector plate. Accordingly, the display function is lost in the night when it is dark outside.
There has been proposed and already known a liquid crystal display watch wherein, in order to overcome such a disadvantage, a plurality of self-luminescence type tubes containing tritium (.sup.3 H) gas and having inner walls coated with a fluorescent material are arrayed underneath the liquid crystal display element, thereby to permit the display in the nighttime.
However, the technique of enclosing the tritium gas into a tube is very difficult (refer to U.S. Pat. No. 3,817,733). Even when the gas is effectively enclosed and the tube is sealed, the gas leaks in due time and is feared to incur radioactive contamination. In addition, since several tubes are arrayed, the brightness at the boundary area between the adjacent tubes becomes low, and the light source exhibits a nonuniform brightness as a whole. A transmission type light scattering member may be placed on the tubes in order to avoid the drawback, but this inevitably renders the self-luminescent light source thick.
To the end of overcoming such disadvantages, it is considered that promethium (.sup.147 Pm) with which the manufacturing stage of work is comparatively easy and which is not feared to leak because it is solid is applied as the self-luminescent fluorescent material. However, .sup.147 Pm gives out high-energy beta rays of 220 KeV. When a substance adapted to absorb the beta rays is interposed, bremsstrahlung is generated, and danger is sometimes involved in case of use in close contact with the human body as in a liquid crystal display wrist watch.